Solvent removal process

ABSTRACT

A process for removing a solvent from a first solution, said process comprising positioning a selective membrane between the first solution and a second solution having a higher osmotic potential than the first solution, such that solvent from the first solution passes across the membrane to dilute the second solution, and extracting solvent from the second solution, wherein the membrane has an average pore size of at least 10 Angstroms, and wherein the second solution contains solute species that are too large to pass through the pores of the membrane.

The present invention relates to a process for removing a solvent from asolution. In particular, although not exclusively, the present inventionrelates to a process for removing water from an aqueous solution, suchas seawater.

Various methods for removing solvents from solutions are known. Forexample, water may be extracted from seawater by distillation methodssuch as multi-stage flash distillation. In a multi-stage flashdistillation process, seawater is introduced into a series of tubes andheated to an elevated temperature. The heated seawater is thenintroduced into an evaporation chamber and subjected to a pressure belowits vapour pressure. The sudden reduction in pressure causes boiling orflashing to occur. The flashed vapours are separated from the saltyresidue by condensation on the tubes of the incoming seawater streams. Aseries of evaporation chambers are employed. Thus, the evaporation orflashing step occurs in multiple stages.

Water may also be separated from seawater by reverse osmosis. In reverseosmosis, seawater is placed on one side of a semi-permeable membrane andsubjected to pressures of 5 to 8 MPa. The other side of the membrane ismaintained at atmospheric pressure. The resulting pressure differentialcauses water to flow across the membrane, leaving a salty concentrate onthe pressurized side of the membrane. Typically, the semi-permeablemembranes have an average pore size of, for example, 1 to 5 Angstroms.

After a period of operation, the pores of the semi-permeable membranemay become obstructed by deposited salts, biological contaminants andsuspended particles in the seawater. Thus, higher pressures may berequired to maintain the desired level of flow across the membrane. Theincreased pressure differential may encourage further clogging to occur.Thus, the membranes must be cleaned and replaced at regular intervals,interrupting the continuity of the process and increasing operationalcosts.

Attempts have been made to reduce the level of clogging of the membrane.For example, the seawater may be pretreated to remove suspendedparticles and biological matter. Alternatively or additionally, theresidual solution on the high-pressure side of the membrane may bedischarged at regular intervals to prevent the osmotic pressure fromexceeding a predetermined threshold.

According to a first aspect of the present invention, there is provideda process for removing a solvent from a first solution, said processcomprising

positioning a selective membrane between the first solution and a secondsolution having a higher osmotic potential than the first solution, suchthat solvent from the first solution passes across the membrane todilute the second solution, and

extracting solvent from the second solution,

wherein the membrane has an average pore size of at least 10 Angstroms,

wherein the second solution contains solute species that are too largeto pass through the pores of the membrane.

Preferably, the membrane has an average pore size of from 10 to 80Angstroms, more preferably, 15 to 50 Angstroms. In a preferredembodiment, the membrane has an average pore size of from 20 to 30Angstroms. The pore size of the membrane may be selected depending onthe size of the solvent molecules that require separation. In general,the larger the pore size, the greater the flux through the membrane.

Any suitable selective membrane may be used in the process of thepresent invention. An array of membranes may be employed. Suitableselective membranes include integral membranes and composite membranes.Specific examples of suitable membranes include membranes formed ofcellulose acetate (CA) and membranes formed of polyamide (PA).Preferably, the membrane is an ion-selective membrane.

The membrane may be planar or take the form of a tube or hollow fibre.If desired, the membrane may be supported on a supporting structure,such as a mesh support. The membrane may be corrugated or of a tortuousconfiguration.

In one embodiment, one or more tubular membranes may be disposed withina housing or shell. The first solution may be introduced into thehousing, whilst the second solution may be introduced into the tubes. Asthe osmotic potential of the first solution is lower than that of thesecond solution, solvent will diffuse across the membrane from the firstsolution into the second solution. Thus, the second solution will becomeincreasingly diluted and the first solution, increasingly concentrated.The diluted second solution may be recovered from the interior of thetubes, whilst the concentrated first solution may be removed from thehousing.

When a planar membrane is employed, the sheet may be rolled such that itdefines a spiral in cross-section.

One or more solutes may be present in each of the solutions. In apreferred embodiment, the first solution comprises a plurality ofsolutes, whilst the second solution is formed by dissolving one or moreknown solutes in a solvent.

In the process of the present invention, the first solution is placed onone side of a selective membrane. A second solution having a higherosmotic potential is placed on the opposite side of the membrane.Typically, although not exclusively, the second solution has a highersolute concentration (and therefore lower solvent concentration) thanfirst solution.

As a result of the difference in osmotic potential between the firstsolution and the second solution, solvent passes across the membranefrom the side of low osmotic potential to the side of high osmoticpotential. The flow occurs with a high flux due to the large averagepore size of the membrane. High pressures are not required to inducesolvent flow. However, a pressure differential across the membrane maybe applied, for example, to enhance the speed of the separation process.

Although the solute species in the first solution may be sufficientlysmall to pass through the pores of the membrane, they are prevented fromdoing so at least initially because of the high osmotic potential on theother side of the membrane. The flow of these solute species across themembrane is only possible once the osmotic potential is equal on bothsides of the membrane or the osmotic potential is higher in the firstsolution.

The second solution contains solute species that are too large to passthrough the pores of the membrane. As a result, solvent from the firstsolution will diffuse into the second solution at a high rate, whilstthe passage of solute between the two solutions is restricted orprevented.

Optionally, the second solution may also contain solute species that aresufficiently small to pass through the pores of the membrane. Thesesmall species will not pass across the membrane if their concentrationin the second solution is below their concentration in the firstsolution. Thus, in a preferred embodiment, the second solutionoptionally contains at least one solute species that is sufficientlysmall to pass through the pores of the membrane in a concentration thatis less than the concentration of the corresponding species in the firstsolution.

As solvent passes from the first solution into the second solution, thefirst solution becomes increasingly concentrated. Once the concentrationof the first solution reaches a certain threshold, the solution may berecovered or discarded. Thus, the process of the present invention maybe used to convert the first solution into a concentrated form fordisposal. Alternatively, further solvent may be removed from theconcentrated first solution by repeating the initial membrane separationstep. Specifically, further solvent may be removed from the concentratedfirst solution by placing this solution on one side of a semi-permeablemembrane. A further solution having an osmotic potential that is higherthan that of the concentrated solution may be placed on the oppositeside of the membrane, such that solvent from the concentrated firstsolution passes across the membrane into the further solution. Thefurther solution may contain the same solute(s) and solvent(s) as thesecond solution. Alternatively, the further solution may containdifferent components.

After solvent from the first solution has passed into the secondsolution, the second solution may be recovered. The second solution maybe at an elevated pressure, even when a pressure is not applied toinduce solvent flow from the first solution to the second solution. Thisis because the flow of solvent from the first solution into the secondsolution occurs along a concentration gradient. This pressure may beused to aid the subsequent extraction of solvent from the secondsolution. For example, when solvent is extracted from the secondsolution by thermal methods, such as multi-stage flash distillation(MSF), the pressure of the second solution may be used to supplement thepumping of the second solution to the multi-stage flash distillationunit. When solvent is extracted from the second solution by membranemethods, such as nanofiltration and reverse osmosis, the pressure of thesecond solution may be used to supplement the pressure applied to thesecond solution to induce solvent flow from the second solution acrossthe selectively permeable membrane. Valves and other pressure regulatingdevices may be used to control the pressure accordingly. One or morepumps may also be used to supplement the pressure of the process streamsif necessary.

The initial flux of solvent across the membrane may be 2 to 80 lm⁻²hr⁻¹,preferably, 5 to 40 lm⁻²hr⁻¹, for example, 15 to 20 lm⁻²hr⁻¹, even inthe absence of an applied pressure on the first solution. However, theflux may vary depending on a number of factors such as the concentrationgradient of the two solutions across the membrane.

The fluid velocity across the surface of the membrane may be varied asrequired to reduce the risk of fouling of the membrane. Generally, thegreater the fluid velocity across the surface of the membrane, the lowerthe risk of fouling.

Solvent may be extracted from the second solution using any suitablemethod. For example, the solvent may be extracted by thermal/pressuremethods (e.g. crystallization and distillation) or using a membrane.Suitable membrane methods include reverse osmosis, nanofiltration,electrodialysis reversal and ion exchange. When reverse osmosis isemployed, the same type of membrane employed in the direct osmosis stepmay be used in the reverse osmosis step. Solvent may be extracted fromthe second solution using hybrid methods combining, for example, thermaland membrane methods of separation.

In a preferred embodiment, nanofiltration membranes are employed toextract solvent from the second solution.

Nanofiltration is particularly suitable for separating the large solutespecies of the second solution from the remainder of the solution.

Suitable nanofiltration membranes include crosslinked polyamidemembranes, such as crosslinked aromatic polyamide membranes. Themembranes may be cast as a “skin layer” on top of a support formed, forexample, of a microporous polymer sheet. The resulting membrane has acomposite structure (e.g. a thin-film composite structure). Typically,the separation properties of the membrane are controlled by the poresize and electrical charge of the “skin layer”. The membranes may besuitable for the separation of components that are 0.01 to 0.001 micronsin size and molecular weights of 100 gmol⁻¹ or above, for example, 200gmol⁻¹ and above.

As well as filtering particles according to size, nanofiltrationmembranes can also filter particles according to their electrostaticproperties. For example, in certain embodiments, the surface charge ofthe nanofiltration membrane may be controlled to provide desiredfiltration properties. For example, the inside of at least some of thepores of the nanofiltration membrane may be negatively charged,restricting or preventing the passage of anionic species, particularlymultivalent anions.

Examples of suitable nanofiltration membranes include Desal-5(Desalination Systems, Escondido, Calif.), NF 70, NF 50, NF 40 and NF 40HF membranes (FilmTech Corp., Minneapolis, Minn.), SU 600 membrane(Toray, Japan) and NRT 7450 and NTR 7250 membranes (Nitto Electric,Japan).

The nanofiltration membranes may be packed as membrane modules. Spiralwound membranes, and tubular membranes, for example, enclosed in a shellmay be employed.

Alternatively, the membranes may be provided as a plate or in a frame.

A multi-stage flash distillation method (MSF) may also be employed toextract solvent from the second solution. For example, the secondsolution may be heated and introduced into an evaporation chamber, whereit is subjected to a pressure below its vapour pressure. The suddenreduction in pressure causes boiling or flashing to occur. The flashedvapours may be separated from the remainder of the solution bycondensation. A series of evaporation chambers are preferably employed.Thus, the evaporation or flashing step can take place in multiplestages. In a preferred embodiment, heat energy from the flashed vapoursis transferred to the incoming solution by heat exchange. As a result ofthis transfer of heat, the vapours are condensed and the temperature ofthe incoming solution increased.

Multiple effect distillation (MED) may also be employed to extractsolvent from the second solution. Multiple effect distillation takesplace in a series of effects and uses the principle of reducing theambient pressure in the various effects. This permits the secondsolution to boil in a series of stages without the need for additionalheat to be supplied after the first effect.

In multiple effect distillation, the second solution may be preheatedand sprayed onto the surface of evaporator tubes as a thin film ofliquid. The tubes are heated by passing a steam through the tubes. Thus,on coming into contact with the heated surface of the tubes, the sprayedliquid evaporates. This vapour is used to heat the evaporator tubes ofthe next effect and the transfer of heat causes the vapour in the tubesto condense. By evaporating and condensing the second solution in thismanner, the solvent from the second solution may be recovered.

The efficiency of the multiple effect distillation step may be increasedby compressing the vapour of at least one of the effects. Thecombination of multiple effect distillation and compression is known asMED-thermo compression.

Mechanical vapour compression (MVD) may also be used to extract solventfrom the second solution. In mechanical vapour compression, vapour froma vessel is typically extracted and then condensed by compression in atube located within the vessel. The compression and condensation stepgenerates heat, which heats the walls of the tube. When second solutionis sprayed onto the surface of the tube, it evaporates generating morevapour. By repeating the extraction, compression and condensation steps,further solvent may be recovered from the second solution.

Rapid spray desalination may also be used to extract the solvent fromthe second solution.

A thermal separation unit that separates dissolved solutes bycrystallization may also be employed to extract the solvent from thesecond solution. For example, the second solution may be cooled in athermal separation column such that at least some of the dissolvedsolutes precipitate out of solution. These precipitates may collect atthe bottom of the column, leaving the solution at the top of the columnwith a reduced solute concentration. This “upper” solution may beretrieved, for example, for direct use or for further purification. Onthe other hand, the concentrated solution at the base of the column maybe heated to dissolve any precipitates. The concentrated solution may bediscarded or recycled to be used again as a fresh second solution. Whenusing a crystallization method to separate the solvent from the secondsolution, the second solution is preferably formed using a salt having asolubility in the solvent of the second solution that is sensitive totemperature variation. Examples of such salts include hydrogenphosphatessuch as disodium hydrogenphosphate (Na₂HPO₄.12H₂O).

Solvent may also be recovered from the second solution by introducingmicro-organisms into the second solution to convert the solutesdissolved in the second solution into insoluble products (i.e.bio-desalting). These insoluble products may then be separated from thesolvent, for example, by filtration. Alternatively, a precipitatingagent may be added to the second solution to precipitate the dissolvedsolutes. The precipitates may then be removed from the solvent by, forexample, filtration.

A combination of techniques may be used to extract the solvent from thesecond solution. In one embodiment, at least two of the followingtechniques are employed to extract the solvent from the second solution:crystallization, distillation, nanofiltration and reverse osmosis. Wheredistillation techniques are employed, at least one of multi-stage flashdistillation, multiple effect distillation, mechanical vapourcompression and rapid spray distillation may be employed. When more thanone technique is used to extract the solvent from the second solution,the techniques may be carried out sequentially or in parallel.

In one embodiment, the second solution may be divided into two portions.The first portion may be treated by nanofiltration or reverse osmosis,whilst the second portion may treated by crystallization and/or adistillation technique selected from at least one of multi-stage flashdistillation, multi-effect distillation, mechanical vapour compression,MED-thermo compression and rapid spray distillation. The residue fromthe nanofiltration or reverse osmosis step may be further treated by acrystallization and/or a distillation technique such as multi-flashdistillation, multi-effect distillation, mechanical vapour compression,MED-thermo compression and/or rapid spray distillation.

In one example, solvent may be extracted from the second solution bymembrane methods, such as nanofiltration or reverse osmosis, followed bya thermal method such as multi-stage flash distillation (MSF). Thesecond solution emerging from the first membrane method step may be atan elevated osmotic pressure. For example, this pressure may be used tohelp to pump the second solution to the MSF unit.

In another example, solvent may be extracted from the second solution bytwo or more membrane separation steps that may, for example, be carriedout in series. The second solution emerging from the first membrane stepmay be at an elevated osmotic pressure. This pressure may be used toinduce solvent flow from the second solution across a subsequentselective membrane and/or semi-permeable membrane.

The solvent of the second solution may be the same or different to thesolvent of the first solution. When the solvent of the second solutionis different to the solvent of the first solution, the extracted solventwill contain a mixture of solvents from the first solution and thesecond solution. This may be useful, for example, when it is desired toproduce a mixture of two solvents from separate impure solutions.

Preferably, the solvent in the first solution is the same as that of thesecond solution. Thus, the extracted solvent consists essentially of asingle solvent. The extracted solvent may be recovered and used in itsrecovered form, or treated further prior to use. For example, when theextracted solvent is water, the water may be stabilized, for example, bypost-treatment stages.

When solvent is extracted from the second solution, a solid residue orresidual solution generally remains behind. This residue or residualsolution may be discarded.

Alternatively, the solid residue may be recovered and used to make freshsecond solution for extracting solvent from the first solution. Theresidual solution may also be recovered and recycled, for example, foruse as fresh second solution for extracting solvent from the firstsolution. In certain cases, it may be possible to use the residualsolution directly as fresh second solution. In other cases, it may benecessary to pre-treat the residual solution, for example, by varyingits concentration accordingly.

The first solution typically contains a solvent that requirespurification. Thus, the process of the present invention may be used topurify organic solvents, such as hydrocarbons (e.g. aliphatic andaromatic hydrocarbons). Mixtures of organic solvents may be purified.The hydrocarbons may be straight chain, branched and/or cyclic. Examplesinclude, but are not limited to, alkanes, alkenes and alkynes. Thehydrocarbons may be substituted with one or more heteroatoms, forexample, fluorine, chlorine, bromine, iodine, oxygen, sulphur, nitrogen,and/or phosphorus atoms. In one embodiment, oxygenated hydrocarbons,such as aldehydes, ketones, carboxylic acids, ethers, esters, alcoholsand/or their derivatives may be purified. For example, glycol ethers andglycol ether esters may also be employed. Alternatively or additionally,halogenated solvents, such as chlorinated, brominated and/or fluorinatedhydrocarbons may be purified.

The process of the present invention may also be used to purifyinorganic solvents, such as acidic solvents, alkaline solvents andneutral solvents, such as water. Such solvents are preferably present inthe first solution and may, additionally be present in the secondsolution. For example, the process of the present invention may be usedto purify aqueous solutions of acids. Alternatively, the process of thepresent invention may be used to purify alkali solvents, such as aqueoushydroxide solutions.

According to a preferred embodiment of the present invention, theprocess is used to remove impurities from water, such as a salinesolution, such as seawater or brackish water. The removal of saltimpurities from seawater and brackish water is known as desalination.

Alternatively, the process may be used to purify water from a wastestream, such an industrial effluent, agricultural or domestic effluent.Thus, such waste streams may be used as the first solution, orpretreated to form the first solution. Grey water streams, for example,waste washing water (e.g. laundry) and streams from gullies, may also bepurified. In one embodiment, waste water from car washes, launderettesand boiler feed streams at power plants may be purified using theprocess of the present invention for reuse.

The process of the present invention may also be used to purify waterfrom other sources, for example, water from rivers or undergroundsources. Domestic water sources may also be purified by the presentinvention to produce drinking water.

The purified water stream may be used for a variety of applications, forexample, for industrial, agricultural, commercial and domestic use (e.g.to produce drinking water). In one embodiment, the purified water topump oil from oil wells. (For avoidance of doubt, the term “purifiedwater” means water that has been treated by the process of the presentinvention. Thus, the purified water stream may include one or moredissolved salts.) In conventional methods for oil recovery, seawater isinjected into an oil well to force the oil from the well to the surface.The use of seawater, however, causes severe operational and scalingproblems. Moreover, it can be difficult to separate seawater from oil,giving rise to separation problems once the oil is recovered. By using apurified water, these problems may be alleviated and/or eliminated. Inaddition, the purified water may be at an elevated osmotic pressure;this increased pressure may be used to aid pumping the water into theoil well.

The second solution is a solution containing solute species that are toolarge to pass through the pores of the membrane. In one embodiment,substantially all the solute species present in the second solution aretoo large to pass through the pores of the membrane. Suitable solutespecies may have a molecular weight of from 50 to 10000 gmol⁻¹,preferably, 150 to 5000 gmol⁻¹, and more preferably, 200 to 2000 gmol⁻¹.

Suitable solutes for the second solution include organic compounds,biological compounds and/or inorganic compounds.

Suitable organic compounds include hydrocarbons, such as aliphatic andaromatic hydrocarbons. Mixtures of two or more organic compounds may beemployed. The hydrocarbons may be straight chain, branched and/orcyclic. Examples of suitable hydrocarbons include, but are not limitedto, alkanes, alkenes and alkynes. The hydrocarbons may be substitutedwith one or more heteroatoms, for example, fluorine, chlorine, bromine,iodine, oxygen, sulphur, nitrogen, and/or phosphorus atoms. In oneembodiment, oxygenated hydrocarbons, such as aldehydes, ketones,carboxylic acids, ethers, esters, alcohols and/or their derivatives maybe employed. The organic solute species may have a molecular weight offrom 200 to 10000 gmol⁻¹, preferably, 300 to 5000 gmol⁻¹, morepreferably, 400 to 2000 gmol⁻¹ and, even more preferably, 500 to 1000gmol⁻¹.

Suitable biological compounds include proteins, amino acids, nucleicacids, carbohydrates and lipids. Mixtures of two or more biologicalcompounds may be employed. Preferred biological solutes include sugars,such as cane sugar and/or beet sugar. Glucose, fructose and sucrose mayalso be employed. The biological solute species may have a molecularweight of from 100 to 10000 gmol⁻¹, preferably, 300 to 5000 gmol⁻¹, morepreferably, 400 to 2000 gmol⁻¹ and, even more preferably, 500 to 1000gmol⁻¹.

Suitable inorganic compounds include acids, bases and salts. Mixtures oftwo or more inorganic compounds may be employed. In a preferredembodiment, the solute in the second solution is a salt. The salt mayhave cationic and/or anionic species that are larger than the averagepore size of the membrane. Preferably, both the solvated cationic andanionic species of the salt are larger than the average pore size of themembrane.

Suitable cationic species include metal ions and ammonium ions. Suitablemetal ions include ions of Groups I to III metals. Examples of suitableGroup I metal ions include ions of sodium and potassium. Examples ofsuitable Group II metal ions include ions of magnesium, calcium,strontium and barium. Examples of suitable Group II cations include ionsof aluminium. Complex cations may also be employed.

Where ammonium ions are employed, such ions may be substituted, forexample, with alkyl groups, such as C₁ to C₂₀ alkyl groups. Substitutedalkyl groups may also be employed.

The cations preferably have an average diameter of greater than 10Angstroms in solvated (e.g. hydrated) form. Preferably, the metalcations have diameters from 11 to 100 Angstroms, more preferably, from15 to 50 Angstroms in solvated (e.g. hydrated) form.

Suitable anionic species include fluorides, chlorides, bromides,iodides, sulphates, sulphites, sulphides, carbonates,hydrogencarbonates, nitrates, nitrites, nitrides, phosphates,hydrogenphosphates, aluminates, borates, bromates, carbides, chlorides,perchlorates, hypochlorates, chromates, fluorosilicates,fluorosilicates, fluorosulphates, silicates, cyanides and cyanates.

The anions preferably have an average diameter of at least 5 Angstromsin solvated form. Preferably, the anions have diameters from 5 to 50Angstroms, more preferably, from 10 Angstroms to 40 Angstroms, and morepreferably, 20 to 30 Angstroms in solvated form.

Preferred salts include magnesium sulfate (MgSO₄.6H₂O or MgSO₄.7H₂O),magnesium chloride (MgCl₂.6H₂O), sodium sulfate (Na₂SO₄.10H₂O), calciumchloride (CaCl₂.2H₂O or CaCl₂.6H₂O), disodium hydrogenphosphate(Na₂HPO₄.12H₂O). and potassium alum (24H₂O).

Optionally, in addition to solutes that are too large to pass throughthe pores of the membrane, the second solution may also include solutesthat are sufficiently small to pass through the pores of the membrane.Examples of such solutes include alkali metal halides, such as alkalimetal chlorides, for example, as sodium chloride and potassium chloride.

These smaller salts may be separated from the second solution using thesame or a different separation method to that employed to separate thelarger solutes from the second solution. Thus, the smaller solutes maybe separated by membrane methods and thermal methods, such as the onesdescribed above. In one embodiment, the larger solutes are separated bya different method to that employed to separate the smaller solutes fromthe second solution. For example, the larger solutes may be separated bynanofiltration, and the smaller solutes may be separated by at least oneof direct osmosis, reverse osmosis, crystallization and/or distillationtechniques.

The second solution may be formed of an organic and/or inorganicsolvent. Suitable organic solvents include hydrocarbons, such asaliphatic and aromatic hydrocarbons. Mixtures of organic solvents may beemployed. The hydrocarbons may be straight chain, branched and/orcyclic. Examples include, but are not limited to, alkanes, alkenes andalkynes. The hydrocarbons may be substituted with one or moreheteroatoms, for example, fluorine, chlorine, bromine, iodine, oxygen,sulphur, nitrogen, and/or phosphorus atoms. In one embodiment,oxygenated hydrocarbons, such as aldehydes, ketones, carboxylic acids,ethers, esters, alcohols and/or their derivatives may be employed. Forexample, glycol ethers and glycol ether esters may also be employed.Alternatively or additionally, halogenated solvents, such aschlorinated, brominated and/or fluorinated hydrocarbons may be employed.

Suitable inorganic solvents include acidic solvents, alkaline solventsand/or water. Water is preferably employed as solvent in the secondsolution.

Preferably, the second solution has a known composition. For example, inone embodiment, the second solution is formed by introducing a knownquantity of a solute into a known quantity of solvent. Preferably, thesecond solution consists essentially of a selected solute dissolved in aselected solvent. By forming the second solution in this manner, asubstantially clean solution may be produced. Preferably, the secondsolution has a reduced concentration of suspended particles, biologicalmatter and/or other components that may cause fouling of the apparatusused to extract solvent from the second solution. More preferably, thesecond solution is substantially free of such components. Thus, membranetechniques may be used to extract solvent from the second solutionwithout fear of the pores of the membrane being subjected tounacceptably high levels of fouling, for example, by biological matteror suspended particles.

Preferably, the second solution is an aqueous solution of at least onesalt. The salt may be selected from one or more of magnesium sulfate(MgSO₄.6H₂O or MgSO₄.7H₂O), magnesium chloride (MgCl₂.6H₂O), sodiumsulfate (Na₂SO₄.10H₂O), calcium chloride (CaCl₂.2H₂O or CaCl₂.6H₂O),disodium hydrogenphosphate (Na₂HPO₄.12H₂O) and potassium alum (24H₂O).

Alternatively, the second solution is an aqueous solution of sugar, suchas glucose, fructose and/or sucrose. The sugar may be derived from anysuitable source. For example, beet sugar and/or cane sugar may beemployed.

In a preferred embodiment, a saline solution (e.g. seawater or brackishwater) is used as the first solution and is placed on one side of themembrane. A second solution having a higher solute concentration thanthe solute concentration of the saline solution is placed on the otherside of the membrane. Preferably, an aqueous solution of magnesiumsulfate magnesium sulfate (MgSO₄.6H₂O or MgSO₄.7H₂O), magnesium chloride(MgCl₂.6H₂O), sodium sulfate (Na₂SO₄.10H₂O), calcium chloride(CaCl₂.2H₂O or CaCl₂.6H₂O) and disodium hydrogenphosphate(Na₂HPO₄.12H₂O) potassium alum (24H₂O) and/or a sugar, such as glucose,fructose and/or sucrose is employed as the second solution. As mentionedabove, additional solutes, such as those that are sufficiently small topass through the pores of the membrane may be included. Examples of suchsolutes include sodium chloride and potassium chloride.

The second solution may contain chemical additives such as anti-scalingagents, corrosion inhibitors, anti-fouling agents and disinfectants.These additives may be contained in the system, for example, when thesecond solution is circulated in a continuous loop.

The difference in solute concentration on either side of the membrane(or osmotic potential) causes water from the saline solution (e.g.seawater or brackish water) to pass into the second solution by osmosis.As the flow of water occurs along the concentration gradient, highpressures are not required to induce flow. However, a pressuredifferential across the membrane may be used, if desired.

Although the dissolved species in the first solution may be sufficientlysmall to pass through the pores of the membrane, they are prevented fromdoing so because of the high osmotic potential on the other side of themembrane. As the second solution contains solute species that are toolarge to pass through the pores of the membrane, these solute speciesare prevented from passing through to the other side of the membrane. Asa result, water from the first solution will diffuse into the secondsolution at a high rate, whilst the passage of solute between the twosolutions is restricted or prevented.

This separation step may be carried out in the absence of an appliedpressure. Thus, although fouling of the membrane may occur, for example,by biological matter (e.g. seaweed, algae, bacteria, fungi and plankton)and suspended particles (e.g. dirt, soil, mud, silt, organic colloids,silica, precipitates and sand particles) in the saline solution (e.g.seawater or brackish water), the membrane may be cleaned or replacedwithout interrupting an expensive stage of the process. Moreover, asthis separation step may be carried out without pressurization, there isno need to re-pressurize the membrane when re-starting the process.

The flow of water from the saline solution dilutes the second solution.Water is then extracted from the diluted second solution. Suitableextraction techniques include the distillation and membrane methodsdescribed above. As described above, a combination of two or more ofthese extraction techniques may be used.

In one embodiment, reverse osmosis is employed. In reverse osmosis, thesecond solution may be placed on one side of a semi-permeable membrane,and subjected to a high pressure. The other side of the membrane ismaintained at a lower pressure. The resulting pressure differentialcauses solvent (e.g. water) to flow across the membrane, leaving behinda residual solution on the pressurized side of the membrane.

Any selectively membrane may be employed in the reverse osmosis step.For example, conventional semi-permeable membranes and nanofiltrationmembranes may be employed.

The pressure differential employed in reverse osmosis may be about 0.1to 20 MPa, preferably, about 0.5 to 15 MPa, more preferably, about 0.7to 7 MPa, and most preferably, about 1 to 3 MPa. One side of themembrane may be pressurized, whilst the other side may be maintained atatmospheric or a sub-atmospheric pressure. Preferably, only one side ofthe membrane is pressurized. The pressurized side of the membrane may besubjected to a pressure of about 0.1 to 20 MPa, preferably, about 0.5 to15 MPa, more preferably, about 0.7 to 7 MPa, and most preferably, about1 to 3 MPa. It should be understood that the precise pressure requiredwould vary depending, for example, on the relative solute concentrationsof the solutions on either side of the membrane.

As explained above, the second solution may contain a lowerconcentration of components that cause membrane fouling (e.g. biologicalmatter and suspended particles) than the first solution. The secondsolution may also contain chemical additives such as anti-scalingagents, corrosion inhibitors, anti-fouling agents and disinfectants. Insuch embodiments, the pressure required to extract solvent from thediluted second solution by reverse osmosis is generally less than thepressure required to extract solvent from the first solution by reverseosmosis using first solution directly. For example, pressures of 5 to 8MPa are required to desalinate seawater directly by reverse osmosis.

The process of the present invention may be continuous or a batchprocess.

The flow of solvent across a membrane is generally influenced by thermalconditions. Thus, the solutions on either side of the membrane may beheated or cooled, if desired. The solutions may be heated totemperatures of 30 to 100° C., for example, 40 to 80° C. Alternatively,the solutions may be cooled to −20 to 20° C., for example, 7 to 12° C.The solution on one side of the membrane may be heated, while the otherside cooled. The heating or cooling may be carried out on each solutionindependently. Chemical reactions may also be carried out on either sideof the membrane, if desired.

The process of the present invention may further comprise apre-treatment step of removing contaminants, such as suspended particlesand biological matter, from the first solution (e.g. a waste stream,seawater or brackish water). Additionally or alternatively, a thresholdinhibitor to control scaling may be added to the first solution.Pre-treatment steps to alter the pH of the first solution may also beemployed. Where seawater is used as the first solution, deep seawater ispreferably employed as generally contains fewer suspended particles andless biological matter than seawater obtained from the surface of theocean. The process of extracting solvent from the first solution mayoptionally be carried out at surface of the ocean or by the coast.

The osmotic potential of the second solution may be enhanced bymicrowave, laser electromagnetic, electric fields (electro osmosis) andelectrokinetic treatment.

According to a second aspect of the present invention, there is provideda process for removing a solvent from a first solution, said processcomprising

positioning a selective membrane between the first solution and a secondsolution having a higher osmotic potential than the first solution, suchthat solvent from the first solution passes across the membrane todilute the second solution, and

recovering solvent from the second solution by a technique selected fromprecipitation, bio-desalting, multi-stage flash distillation,multi-effect desalination, mechanical vapour compression, rapid spraydesalination and nanofiltration.

Any suitable selective membrane may be used in the process of thepresent invention. The membrane may have an average pore size of 1 to 80Angstroms, preferably, 5 to 70 Angstroms, more preferably, 10 to 65Angstroms, for example, 15 to 50 Angstroms. In one embodiment, themembrane has an average pore size of 20 to 30 Angstroms.

Suitable selective membranes include integral membranes and compositemembranes. Specific examples of suitable membranes include membranesformed of cellulose acetate (CA) and membranes formed of polyamide (PA).Preferably, the membrane is an ion-selective membrane. Conventionalsemi-permeable membranes may also be employed.

The membrane may be planar or take the form of a tube or hollow fibre.If desired, the membrane may be supported on a supporting structure,such as a mesh support. The membrane may be corrugated or of a tortuousconfiguration.

In one embodiment, one or more tubular membranes may be disposed withina housing or shell. The first solution may be introduced into thehousing, whilst the second solution may be introduced into the tubes. Asthe solvent concentration of the first solution is higher than that ofthe second, solvent will diffuse across the membrane from the firstsolution into the second solution. Thus, the second solution will becomeincreasingly diluted and the first solution, increasingly concentrated.The diluted second solution may be recovered from the interior of thetubes, whilst the concentrated first solution may be removed from thehousing.

When a planar membrane is employed, the sheet may be rolled such that itdefines a spiral in cross-section.

One or more solutes may be present in each of the solutions. In apreferred embodiment, the first solution comprises a plurality ofsolutes, whilst the second solution is formed by dissolving one or moreknown solutes in a solvent.

In the process of the present invention, the first solution is placed onone side of a selective membrane. A second solution having a higherosmotic potential is placed on the opposite side of the membrane.Typically, although not exclusively, the second solution has a highersolute concentration (and therefore lower solvent concentration) thanfirst solution.

As a result of the difference in osmotic potential between the firstsolution and the second solution, solvent passes across the membranefrom the side of low osmotic potential to the side of high osmoticpotential. High pressures are not required to induce solvent flow.However, a pressure differential across the membrane may be applied, forexample, to enhance the speed of the separation process.

Although the solute species in the first solution may be sufficientlysmall to pass through the pores of the membrane, they are prevented fromdoing so because of the high osmotic potential on the other side of themembrane.

As solvent passes from the first solution into the second solution, thefirst solution becomes increasingly concentrated. Once the concentrationof the first solution reaches a certain threshold, the solution may berecovered or discarded. Thus, the process of the present invention maybe used to convert the first solution into a concentrated form fordisposal. Alternatively, further solvent may be removed from theconcentrated first solution by repeating the initial membrane separationstep. Specifically, further solvent may be removed from the concentratedfirst solution by placing this solution on one side of a semi-permeablemembrane. A further solution having an osmotic potential that is higherthan that of the concentrated solution may be placed on the oppositeside of the membrane, such that solvent from the concentrated firstsolution passes across the membrane into the further solution. Thefurther solution may contain the same solute(s) and solvent(s) as thesecond solution. Alternatively, the further solution may containdifferent components.

After solvent from the first solution has passed into the secondsolution, the second solution may be recovered. The second solution maybe at an elevated pressure, even when a pressure is not applied toinduce solvent flow from the first solution to the second solution. Thisis because the flow of solvent from the first solution into the secondsolution occurs along a concentration gradient. This pressure may beused to aid the subsequent extraction of solvent from the secondsolution. For example, when solvent is extracted from the secondsolution by membrane methods, such as nanofiltration and reverseosmosis, the pressure of the second solution may be used to supplementthe pressure applied to the second solution to induce solvent flow fromthe second solution across the selectively permeable membrane employed.For example, when solvent is extracted from the second solution bythermal methods, such as multi-stage flash distillation (MSF), thepressure of the second solution may be used to supplement the pumping ofsecond solution to the MSF unit. Valves and other pressure regulatingdevices may be used to control the pressure accordingly. One or morepumps may also be used to supplement the pressure of the process streamsif necessary.

The initial flux of solvent across the membrane may be 2 to 80 lm⁻²hr⁻¹,preferably, 5 to 40 lm⁻²hr⁻¹, for example, 15 to 20 lm⁻²hr⁻¹, even inthe absence of an applied pressure on the first solution. However, theflux may vary depending on a number of factors such as the concentrationgradient of the two solutions across the membrane.

The fluid velocity across the surface of the membrane may be varied asrequired to reduce the risk of fouling of the membrane. Generally, thegreater the fluid velocity across the surface of the membrane, the lowerthe risk of fouling.

Solvent may be extracted from the second solution using any suitablemethod. For example, the solvent may be extracted by thermal/pressuremethods (e.g. distillation) or using a membrane. Suitable membranemethods include reverse osmosis, nanofiltration, electrodialysis,reversal and ion exchange. When reverse osmosis is employed, the sametype of membrane employed in the direct osmosis step may be used in thereverse osmosis step. Solvent may be extracted from the second solutionusing hybrid methods combining, for example, thermal and membranemethods of separation.

In a preferred embodiment, nanofiltration membranes are employed toextract solvent from the second solution. Nanofiltration is particularlysuitable for separating the large solute species of the second solutionfrom the remainder of the solution.

Suitable nanofiltration membranes include crosslinked polyamidemembranes, such as crosslinked aromatic polyamide membranes. Themembranes may be cast as a “skin layer” on top of a support formed, forexample, of a microporous polymer sheet. The resulting membrane has acomposite structure (e.g. a thin-film composite structure). Typically,the separation properties of the membrane are controlled by the poresize and electrical charge of the “skin layer”. The membranes may besuitable for the separation of components that are 0.01 to 0.001 micronsin size and molecular weights of 100 gmol⁻¹ or above, for example, 200gmol⁻¹ and above.

As well as filtering particles according to size, nanofiltrationmembranes can also filter particles according to their electrostaticproperties. For example, in certain embodiments, the surface charge ofthe nanofiltration membrane may be controlled to provide desiredfiltration properties. For example, the inside of at least some of thepores of the nanofiltration membrane may be negatively charged,restricting or preventing the passage of anionic species, particularlymultivalent anions.

Examples of suitable nanofiltration membranes include Desal-5(Desalination Systems, Escondido, Calif.), NF 70, NF 50, NF 40 and NF 40HF membranes (FilmTech Corp., Minneapolis, Minn.), SU 600 membrane(Toray, Japan) and NRT 7450 and NTR 7250 membranes (Nitto Electric,Japan).

The nanofiltration membranes may be packed as membrane modules. Spiralwound membranes, and tubular membranes, for example, enclosed in a shellmay be employed.

Alternatively, the membranes may be provided as a plate or in frame.

A multi-stage flash distillation method (MSF) may also be employed toextract solvent from the second solution. For example, the secondsolution may be heated and introduced into an evaporation chamber, whereit is subjected to a pressure below its vapour pressure. The suddenreduction in pressure causes boiling or flashing to occur. The flashedvapours may be separated from the remainder of the solution bycondensation. A series of evaporation chambers are preferably employed.Thus, the evaporation or flashing step can take place in multiplestages. In a preferred embodiment, heat energy from the flashed vapoursis transferred to the incoming solution by heat exchange. As a result ofthis transfer of heat, the vapours are condensed and the temperature ofthe incoming solution increased.

Multiple effect distillation (MED) may also be employed to extractsolvent from the second solution. Multiple effect distillation takesplace in a series of effects and uses the principle of reducing theambient pressure in the various effects. This permits the secondsolution to boil in a series of stages without the need for additionalheat to be supplied after the first effect.

In multiple effect distillation, the second solution may be preheatedand sprayed onto the surface of evaporator tubes as a thin film ofliquid. The tubes are heated by passing a steam through the tubes. Thus,on coming into contact with the heated surface of the tubes, the sprayedliquid evaporates. This vapour is used to heat the evaporator tubes ofthe next effect and the transfer of heat causes the vapour in the tubesto condense. By evaporating and condensing the second solution in thismanner, the solvent from the second solution may be recovered.

The efficiency of the multiple effect distillation step may be increasedby compressing the vapour of at least one of the effects. Thecombination of multiple effect distillation and compression is known asMED-thermo compression.

Mechanical vapour compression (MVD) may also be used to extract solventfrom the second solution. In mechanical vapour compression, vapour froma vessel is typically extracted and then condensed by compression in atube located within the vessel. The compression and condensation stepgenerates heat, which heats the walls of the tube. When second solutionis sprayed onto the surface of the tube, it evaporates generating morevapour. By repeating the extraction, compression and condensation steps,further solvent may be recovered from the second solution.

Rapid spray desalination may also be used to extract the solvent fromthe second solution.

A thermal separation unit that separates dissolved solutes bycrystallization may also be employed to extract the solvent from thesecond solution. For example, the second solution may be cooled in athermal separation column such that at least some of the dissolvedsolutes precipitate out of solution. These precipitates may collect atthe bottom of the column, leaving the solution at the top of the columnwith a reduced solute concentration. This “upper” solution may beretrieved, for example, for direct use or for further purification. Onthe other hand, the concentrated solution at the base of the column maybe heated to dissolve any precipitates. The concentrated solution may bediscarded or recycled to be used again as a fresh second solution. Whenusing a crystallization method to separate the solvent from the secondsolution, the second solution is preferably formed using a salt having asolubility in the solvent of the second solution that is sensitive totemperature variation. Examples of such salts include hydrogenphosphatessuch as disodium hydrogenphosphate (Na₂HPO₄.12H₂O)

Solvent may also be recovered from the second solution by introducingmicro-organisms into the second solution to convert the solutesdissolved in the second solution into insoluble products (i.e.bio-desalting). These insoluble products may then be separated from thesolvent, for example, by filtration. Alternatively, a precipitatingagent may be added to the second solution to precipitate the dissolvedsolutes. The precipitates may then be removed from the solvent by, forexample, filtration.

A combination of techniques may be used to extract the solvent from thesecond solution. In one embodiment, at least two of the followingtechniques are employed to extract the solvent from the second solution:crystallization, distillation, nanofiltration and reverse osmosis. Wheredistillation techniques are employed, at least one of multi-stage flashdistillation, multiple effect distillation, mechanical vapourcompression and rapid spray distillation may be employed. When more thanone technique is used to extract the solvent from the second solution,the techniques may be carried out sequentially or in parallel.

In one embodiment, the second solution may be divided into two portions.The first portion may be treated by crystallization and/ornanofiltration or reverse osmosis, whilst the second portion may treatedby crystallization and/or a distillation technique selected from atleast one of multi-flash distillation, multi-effect distillation(including MED-thermo compression), mechanical vapour compression, andrapid spray distillation. The residue from the nanofiltration or reverseosmosis step may be further treated by crystallization and/or adistillation technique such as multi-flash distillation, multi-effectdistillation, mechanical vapour compression, and/or rapid spraydistillation.

In another embodiment, solvent may be extracted from the second solutionby a membrane technique followed by crystallization and/or thermaldistillation such as multistage flash distillation, multi-effectdistillation, mechanical vapour compression, and/or rapid spraydistillation. Suitable membrane techniques include nanofiltration andreverse osmosis. A selective membrane having an average pore size of atleast 10 Angstroms may be employed (see first aspect of the presentinvention). Alternatively, a conventional semi-permeable membrane may beused.

The solvent of the second solution may be the same or different to thesolvent of the first solution. When the solvent of the second solutionis different to the solvent of the first solution, the extracted solventwill contain a mixture of solvents from the first solution and thesecond solution. This may be useful, for example, when it is desired toproduce a mixture of two solvents from separate impure solutions.

Preferably, the solvent in the first solution is the same as that of thesecond solution. Thus, the extracted solvent consists essentially of asingle solvent. The extracted solvent may be recovered and used in itsrecovered form, or treated further prior to use. For example, when theextracted solvent is water, the water may be stabilized, for example, bypost-treatment stages.

When solvent is extracted from the second solution, a solid residue orresidual solution generally remains behind. This residue or residualsolution may be discarded.

Alternatively, the solid residue may be recovered and used to make freshsecond solution for extracting solvent from the first solution. Theresidual solution may also be recovered and recycled, for example, foruse as fresh second solution for extracting solvent from the firstsolution. In certain cases, it may be possible to use the residualsolution directly as fresh second solution. In other cases, it may benecessary to pre-treat the residual solution, for example, by varyingits concentration accordingly.

The first solution typically contains a solvent that requirespurification. Thus, the process of the present invention may be used topurify organic solvents, such as hydrocarbons (e.g. aliphatic andaromatic hydrocarbons). Mixtures of organic solvents may be purified.The hydrocarbons may be straight chain, branched and/or cyclic. Examplesinclude, but are not limited to, alkanes, alkenes and alkynes. Thehydrocarbons may be substituted with one or more heteroatoms, forexample, fluorine, chlorine, bromine, iodine, oxygen, sulphur, nitrogen,and/or phosphorus atoms. In one embodiment, oxygenated hydrocarbons,such as aldehydes, ketones, carboxylic acids, ethers, esters, alcoholsand/or their derivatives may be purified. For example, glycol ethers andglycol ether esters may also be employed. Alternatively or additionally,halogenated solvents, such as chlorinated, brominated and/or fluorinatedhydrocarbons may be purified.

The process of the present invention may also be used to purifyinorganic solvents, such as acidic solvents, alkaline solvents andneutral solvents, such as water. Such solvents are preferably present inthe first solution and may, additionally be present in the secondsolution. For example, the process of the present invention may be usedto purify aqueous solutions of acids. Alternatively, the process of thepresent invention may be used to purify alkali solvents, such as aqueoushydroxide solutions.

According to a preferred embodiment of the present invention, theprocess is used to remove impurities from water, such as a salinesolution, such as seawater or brackish water. The removal of saltimpurities from seawater and brackish water is known as desalination.Alternatively, the process may be used to purify water from a wastestream, such an industrial effluent, domestic or agricultural effluent.Thus, such waste streams may be used as the first solution, orpretreated to form the first solution. Grey water streams, for example,waste washing water (e.g. laundry) and streams from gullies, may also bepurified. In one embodiment, waste water from car washes, launderettesand boiler feed streams at power plants may be purified using theprocess of the present invention for reuse.

The process of the present invention may also be used to purify waterfrom other sources, for example, water from rivers or undergroundsources. Domestic water sources may also be purified using the method ofthe present invention to produce drinking water.

The purified water stream may be used for a variety of applications, forexample, for industrial, agricultural, commercial and domestic use (e.g.to produce drinking water). In one embodiment, the purified water topump oil from oil wells. (For avoidance of doubt, the term “purifiedwater” means water that has been treated by the process of the presentinvention. Thus, the purified water stream may include one or moredissolved salts.) In conventional methods for oil recovery, seawater isinjected into an oil well to force the oil from the well to the surface.The use of seawater, however, causes severe operational and scalingproblems. Moreover, it can be difficult to separate seawater from oil,giving rise to separation problems once the oil is recovered. By using apurified water, these problems may be alleviated and/or eliminated. Inaddition, the purified water may be at an elevated osmotic pressure;this increased pressure may be used to aid pumping the water into theoil well.

The second solution is a solution containing solute species that are toolarge to pass through the pores of the membrane. In one embodiment,substantially all the solute species present in the second solution aretoo large to pass through the pores of the membrane. Suitable solutespecies may have a molecular weight of from 50 to 10000 gmol⁻¹,preferably, 100 to 5000 gmol⁻¹, and more preferably, 200 to 2000 gmol⁻¹.

Suitable solutes for the second solution include organic compounds,biological compounds and/or inorganic compounds.

Suitable organic compounds include hydrocarbons, such as aliphatic andaromatic hydrocarbons. Mixtures of two or more organic compounds may beemployed. The hydrocarbons may be straight chain, branched and/orcyclic. Examples of suitable hydrocarbons include, but are not limitedto, alkanes, alkenes and alkynes. The hydrocarbons may be substitutedwith one or more heteroatoms, for example, fluorine, chlorine, bromine,iodine, oxygen, sulphur, nitrogen, and/or phosphorus atoms. In oneembodiment, oxygenated hydrocarbons, such as aldehydes, ketones,carboxylic acids, ethers, esters, alcohols and/or their derivatives maybe employed. The organic solute species may have a molecular weight offrom 100 to 10000 gmol⁻¹, preferably, 300 to 5000 gmol⁻¹, morepreferably, 400 to 2000 gmol⁻¹ and, even more preferably, 500 to 1000gmol⁻¹.

Suitable biological compounds include proteins, amino acids, nucleicacids, carbohydrates and lipids. Mixtures of two or more biologicalcompounds may be employed. Preferred biological solutes include sugars,such as cane sugar and/or beet sugar. Glucose, fructose and sucrose mayalso be employed. The biological solute species may have a molecularweight of from 100 to 10000 gmol⁻¹, preferably, 300 to 5000 gmol⁻¹, morepreferably, 400 to 2000 gmol⁻¹ and, even more preferably, 500 to 1000gmol⁻¹.

Suitable inorganic compounds include acids, bases and salts. Mixtures oftwo or more inorganic compounds may be employed. In a preferredembodiment, the solute in the second solution is a salt. The salt mayhave cationic and/or anionic species that are larger than the averagepore size of the membrane. Preferably, both the solvated cationic andanionic species of the salt are larger than the average pore size of themembrane.

Suitable cationic species include metal ions and ammonium ions. Suitablemetal ions include ions of Groups I to III metals. Examples of suitableGroup I metal ions include ions of sodium and potassium. Examples ofsuitable Group II metal ions include ions of magnesium, calcium,strontium and barium. Examples of suitable Group II cations include ionsof aluminium. Complex cations may also be employed.

Where ammonium ions are employed, such ions may be substituted, forexample, with alkyl groups, such as C₁ to C₂₀ alkyl groups. Substitutedalkyl groups may also be employed.

Suitable anionic species include fluorides, chlorides, bromides,iodides, sulphates, sulphites, sulphides, carbonates,hydrogencarbonates, nitrates, nitrites, nitrides, phosphates,aluminates, borates, bromates, carbides, chlorides, hydrogenphosphates,perchlorates, hypochlorates, chromates, fluorosilicates,fluorosilicates, fluorosulphates, silicates, cyanides and cyanates.

Preferred salts include magnesium sulfate (MgSO₄.6H₂O or MgSO₄.7H₂O),magnesium chloride (MgCl₂.6H₂O), sodium sulfate (Na₂SO₄.10H₂O), calciumchloride (CaCl₂.2H₂O or CaCl₂.6H₂O), potassium alum (24H₂O), disodiumhydrogenphosphate (Na₂HPO₄.12H₂), sodium chloride(NaCl) and potassiumchloride (KCl).

The second solution may be formed of an organic and/or inorganicsolvent. Suitable organic solvents include hydrocarbons, such asaliphatic and aromatic hydrocarbons. Mixtures of organic solvents may beemployed. The hydrocarbons may be straight chain, branched and/orcyclic. Examples include, but are not limited to, alkanes, alkenes andalkynes. The hydrocarbons may be substituted with one or moreheteroatoms, for example, fluorine, chlorine, bromine, iodine, oxygen,sulphur, nitrogen, and/or phosphorus atoms. In one embodiment,oxygenated hydrocarbons, such as aldehydes, ketones, carboxylic acids,ethers, esters, alcohols and/or their derivatives may be employed. Forexample, glycol ethers and glycol ether esters may also be employed.Alternatively or additionally, halogenated solvents, such aschlorinated, brominated and/or fluorinated hydrocarbons may be employed.

Suitable inorganic solvents include acidic solvents, alkaline solventsand/or water. Water is preferably employed as solvent in the secondsolution.

Preferably, the second solution has a known composition. For example, inone embodiment, the second solution is formed by introducing a knownquantity of a solute into a known quantity of solvent. Preferably, thesecond solution consists essentially of a selected solute dissolved in aselected solvent. By forming the second solution in this manner, asubstantially clean solution may be produced. Preferably, the secondsolution has a reduced concentration of suspended particles, biologicalmatter and/or other components that may cause fouling of the apparatusused to extract solvent from the second solution. More preferably, thesecond solution is substantially free of such components. Thus, membranetechniques may be used to extract solvent from the second solutionwithout fear of the pores of the membrane being subjected tounacceptably high levels of fouling, for example, by biological matteror suspended particles.

Preferably, the second solution is an aqueous solution of at least onesalt. The salt may be selected from one or more of sodium chloride,potassium chloride, magnesium sulfate (MgSO₄.6H₂O or MgSO₄.7H₂O),magnesium chloride (MgCl₂.6H₂O), sodium sulfate (Na₂SO₄.10H₂O), calciumchloride (CaCl₂.2H₂O or CaCl₂.6H₂O), disodium hydrogenphosphate(Na₂HPO₄.12H₂O) and potassium alum (24H₂O). Alternatively, the secondsolution is an aqueous solution of sugar, such as glucose, fructoseand/or sucrose. The sugar may be derived from any suitable source. Forexample, beet sugar and/or cane sugar may be employed.

In a preferred embodiment, a saline solution (e.g. seawater or brackishwater) is used as the first solution and is placed on one side of themembrane. A second solution having a higher solute concentration thanthe solute concentration of the saline solution is placed on the otherside of the membrane. Preferably, an aqueous solution of sodium chloride(NaCl), potassium chloride (KCl), magnesium sulfate (MgSO₄.6H₂O orMgSO₄.7H₂O), magnesium chloride (MgCl₂.6H₂O), sodium sulfate(Na₂SO₄.10H₂O), calcium chloride (CaCl₂.2H₂O or CaCl₂.6H₂O), disodiumhydrogenphosphate (Na₂HPO₄.12H₂O) and potassium alum (24H₂O) and/or asugar, such as glucose, fructose and/or sucrose is employed as thesecond solution.

The second solution may contain chemical additives such as anti-scalingagents, corrosion inhibitors, anti-fouling agents and disinfectants. Theadditives may be contained in the system and reused, for example, whenthe second solution is circulated in a continuous loop.

The difference in osmotic potential on either side of the membranecauses water from the saline solution (e.g. seawater or brackish water)to pass into the second solution by osmosis. As the flow of water occursalong the concentration gradient, high pressures are not required toinduce flow. However, a pressure differential across the membrane may beused, if desired.

Although the dissolved species in the first solution may be sufficientlysmall to pass through the pores of the membrane, they are prevented fromdoing so because of the high osmotic potential on the other side of themembrane.

The initial membrane separation step may be carried out in the absenceof an applied pressure. Thus, although fouling of the membrane mayoccur, for example, by biological matter (e.g. seaweed, algae, bacteria,fungi and plankton) and suspended particles (e.g. dirt, soil, mud, silt,organic colloids, silica, precipitates and sand particles) in the salinesolution (e.g. seawater or brackish water), the membrane may be cleanedor replaced without interrupting an expensive stage of the process.Moreover, as this separation step may be carried out withoutpressurization, there is no need to re-pressurize the membrane whenre-starting the process.

The flow of water from the saline solution dilutes the second solution.Water is then extracted from the diluted second solution. Suitableextraction techniques include the distillation and membrane methodsdescribed above. As described above, a combination of two or more ofthese extraction techniques may be used.

In one embodiment, reverse osmosis is employed. In reverse osmosis, thesecond solution may be placed on one side of a semi-permeable membrane,and subjected to a high pressure.

The other side of the membrane is maintained at a lower pressure. Theresulting pressure differential causes solvent (e.g. water) to flowacross the membrane, leaving behind a residual solution on thepressurized side of the membrane.

Any selectively membrane may be employed in the reverse osmosis step.For example, conventional semi-permeable membranes and nanofiltrationmembranes may be employed.

The pressure differential employed in reverse osmosis may be about 0.1to 20 MPa, preferably, about 0.5 to 15 MPa, more preferably, about 0.7to 7 MPa, and most preferably, about 1 to 3 MPa. One side of themembrane may be pressurized, whilst the other side may be maintained atatmospheric or a sub-atmospheric pressure. Preferably, only one side ofthe membrane is pressurized. The pressurized side of the membrane may besubjected to a pressure of about 0.1 to 20 MPa, preferably, about 0.5 to15 MPa, more preferably, about 0.7 to 7 MPa, and most preferably, about1 to 3 MPa. It should be understood that the precise pressure requiredwould vary depending, for example, on the relative solute concentrationsof the solutions on either side of the membrane.

As explained above, the second solution may contain a lowerconcentration of components that cause membrane fouling (e.g. biologicalmatter and suspended particles) than the first solution. The secondsolution may also contain chemical additives such as anti-scalingagents, corrosion inhibitors, anti-fouling agents and disinfectants. Insuch embodiments, the pressure required to extract solvent from thediluted second solution by reverse osmosis is generally less than thepressure required to extract solvent from the first solution by reverseosmosis using first solution directly. For example, pressures of 5 to 8MPa are required to desalinate seawater directly by reverse osmosis.

The process of the present invention may be continuous or a batchprocess.

The flow of solvent across a membrane is generally influenced by thermalconditions. Thus, the solutions on either side of the membrane may beheated or cooled, if desired. The solutions may be heated totemperatures of 30 to 100° C., for example, 40 to 80° C. Alternatively,the solutions may be cooled to −20 to 20° C., for example, 7 to 12° C.The solution on one side of the membrane may be heated, while the otherside cooled. The heating or cooling may be carried out on each solutionindependently. Chemical reactions may also be carried out on either sideof the membrane, if desired.

The process of the present invention may further comprise apre-treatment step of removing contaminants, such as suspended particlesand biological matter, from the first solution (e.g. a waste stream,seawater or brackish water). Additionally or alternatively, a thresholdinhibitor to control scaling may be added to the first solution.Pre-treatment steps to alter the pH of the first solution may also beemployed. Where seawater is used as the first solution, deep seawater ispreferably employed as generally contains fewer suspended particles andless biological matter than seawater obtained from the surface of theocean.

The process of extracting solvent from the first solution may optionallybe carried out at surface of the ocean or by the coast.

The osmotic potential of the second solution may be enhanced bymicrowave, laser electromagnetic, electric fields (electro osmosis) andelectrokinetic treatment.

The processes of the first and second aspect of the present inventionmay be used to concentrate the first solution. For example, the firstsolution may be concentrated into a form that is more convenient fordisposal (reduced volume). This may be useful if the first solution is awaste stream.

Preferred embodiments of the process of the present invention will nowbe described, by way of example, with reference to the accompanyingdrawings, in which:

FIG. 1 is a schematic flow diagram of an apparatus for desalinatingseawater by a conventional reverse osmosis process,

FIG. 2 is a schematic flow diagram of an apparatus for desalinatingseawater using a process according to a first embodiment of the presentinvention,

FIG. 3 is a schematic flow diagram of an apparatus for desalinatingseawater using a process according to a second embodiment of the presentinvention,

FIG. 4 is a schematic flow diagram of an apparatus for desalinatingseawater using a process according to a third embodiment of the presentinvention,

FIG. 5 is a schematic flow diagram of an apparatus for desalinatingseawater using a process according to a fourth embodiment of the presentinvention,

FIG. 6 is a schematic flow diagram of an apparatus for desalinatingseawater using a process according to a fifth embodiment of the presentinvention,

FIG. 7 is a schematic flow diagram of an apparatus for desalinatingseawater using a process according to a sixth embodiment of the presentinvention,

FIG. 8 is a schematic flow diagram of an apparatus for desalinatingseawater using a process according to a seventh embodiment of thepresent invention,

FIG. 9 is a schematic flow diagram of an apparatus for desalinatingseawater using a process according to an eighth embodiment of thepresent invention, and

FIG. 10 is a schematic flow diagram of an apparatus for desalinatingseawater using a process according to a ninth embodiment of the presentinvention.

Reference is first made to FIG. 1 of the drawings. This Figure depictsan apparatus 10 for performing a conventional desalination process byreverse osmosis. The apparatus 10 comprises a high-pressure pump 12 anda membrane module 14. The module 14 contains a semi-permeable membrane16.

In use, seawater is pumped into the module 14 using the high-pressurepump 12. This causes the seawater to come into contact with the one sideof the semi-permeable membrane 16 at high pressure. Typically, pressuresof 5 to 8 MPa are employed. As a result, water flows through themembrane 16, leaving a concentrated seawater solution on the pressurizedside of the membrane 16. The concentrated seawater solution may beremoved and discarded via line 18.

The water collected on the unpressurized side of the membrane 16 issubstantially pure, and is removed from the module 14 via line 20.

After a period of use, the semi-permeable membrane 16 becomes clogged bydeposits and suspended particles in the seawater. Thus, the reverseosmosis step has to be stopped about every two to four months to cleanand/or replace the membrane 16.

Reference is now made to FIG. 2 of the drawings, which depicts anapparatus for desalinating seawater using a process according to a firstembodiment of the present invention.

The apparatus 100 comprises a first membrane module 110 and a secondmembrane module 112. Each of the modules 110, 112 contains a membrane114 a, 114 b. The first membrane 114 a is an ion-selective membranehaving an average pore size of 10 Angstroms. The second membrane 114 bis a nanofiltration membrane.

The first membrane module 110 is coupled to a storage tank 116. Thestorage tank 116 is coupled to the second membrane module 112 via a pump118. The apparatus 100 also comprises a mixing tank 120 for producing asolution of magnesium sulfate.

A magnesium sulfate solution is formed in mixing tank 120 by dissolvinga known quantity of magnesium sulfate in water. The resulting solutionhas a magnesium sulfate concentration that is higher than the totaldissolved salt (TDS) concentration of the seawater under treatment.

Seawater is introduced to one side of the membrane 114 a of the firstmembrane module 110 via line 122. The magnesium sulfate solution isintroduced to the other side of the membrane 114 a. As the magnesiumsulfate solution has a solute concentration that is higher than thetotal dissolved salt (TDS) concentration of seawater, water flows acrossthe membrane 114 a by direct osmosis. The flow of water dilutes themagnesium sulfate solution, leaving behind a salty residual solution onthe seawater side of the membrane 114 a. The latter may be removed vialine 124.

Magnesium and sulfate ions are too large to pass through the pores ofthe membrane 114 a. Thus, there is no back flow of solute from themagnesium sulfate solution into the seawater.

The diluted magnesium sulfate solution is recovered from the firstmembrane module 110 and transferred to the storage tank 116. The dilutedmagnesium sulfate solution is then transferred to the second membranemodule 112 using the pump 118.

In the second membrane module 112, the diluted magnesium sulfatesolution is introduced into the membrane module 112 where it iscontacted with one side of a nanofiltration membrane 114 b. Themagnesium and sulfate ions in the diluted magnesium sulfate solution aretoo large to pass through the pores of the membrane 114 b and areretained on the membrane as a residue. This residue can be recycled tostorage tank 120 via line 126.

Water from the magnesium sulfate solution, on the other hand, passesthrough the nanofiltration membrane 114 b and this is recovered via line128.

Reference is now made to FIG. 3 of the drawings, which depicts anapparatus for desalinating seawater using a process according to asecond embodiment of the present invention. The embodiment of FIG. 3 issimilar to that of FIG. 2 and like numerals have been used to illustratelike parts. The apparatus of FIG. 3, however, further includes third andfourth membrane modules 130, 132.

A solution of magnesium sulfate and sodium chloride is formed in mixingtank 120. The total dissolved salt (TDS) concentration of the resultingsolution is higher than the TDS of the seawater under treatment.

Seawater is introduced to one side of the membrane 114 a of the firstmembrane module 110 via line 122. The magnesium sulfate/sodium chloridesolution is introduced to the other side of the membrane 114 a. As themagnesium sulfate/sodium chloride solution has a solute concentrationthat is higher than the total dissolved salt (TDS) concentration ofseawater, water flows across the membrane 114 a by direct osmosis. Theflow of water dilutes the magnesium sulfate/sodium chloride solution,leaving behind a salty residual solution on the seawater side of themembrane 114 a. The latter may be removed via line 124.

The diluted magnesium sulfate/sodium chloride solution is recovered fromthe first membrane module 110 and transferred to the second membranemodule 112 using the pump 118.

In the second membrane module 112, the diluted magnesium sulfate/sodiumchloride solution is passed through a nanofiltration membrane 114 b. Asmagnesium and sulfate ions are too large to pass through the pores ofthe membrane, these retained by the nanofiltration membrane 114 b as aresidue. This residue is recycled to mixing tank 120 via line 126.Sodium and chloride ions, on the other hand, are sufficiently small topass through the pores of the nanofiltration membrane 114 b. Thus,sodium chloride solution is collected as a filtrate, which istransferred to the third membrane module 130 for further treatment.

In the third membrane module 130, the sodium chloride solution iscontacted with one side of a semi-permeable membrane. A magnesiumsulfate solution having a higher total dissolved salts concentration iscontacted with the other side of the membrane. As a result of thedifference in osmotic pressure across the membrane, water flows acrossthe membrane to dilute the magnesium sulfate solution. The sodiumchloride solution on the other side of the membrane becomes increasinglyconcentrated and is recycled to the storage tank 120.

The diluted magnesium sulfate solution is retrieved from the thirdmembrane module and introduced into the fourth membrane module 132. Inthe fourth membrane module, the diluted magnesium sulfate solution ispassed through a nanofiltration membrane. As magnesium and sulfate ionsare too large to pass through the membrane, they are retained by thenanofiltration membrane as a residue. This residue may be recycled tothe third membrane module 130.

The diluted magnesium sulfate solution introduced into the fourthmembrane module 132 may be at an elevated pressure, due to the influx ofwater from the sodium chloride solution This elevated pressure may helpto push the magnesium sulfate solution across the nanofiltrationmembrane in the fourth membrane module 132. Alternatively oradditionally, a pump may be used to aid the passage of liquid throughthe membrane.

The water that passes through the nanofiltration module is substantiallypure and this may be removed via line 134.

Reference is now made to FIG. 4 of the drawings, which depicts anapparatus for desalinating seawater using a process according to a thirdembodiment of the present invention.

The apparatus 200 comprises a membrane module 210 and a multi-stageflash distillation unit 212. The membrane module 210 contains aconventional semi-permeable membrane 214.

Seawater 216 is introduced to one side of the membrane 214. The otherside of the membrane 214 is in contact with a solution of magnesiumsulfate 218 having a higher total dissolved salts concentration thanseawater 216. The difference in osmotic potential causes water to flowacross the membrane 214 by direct osmosis. The flow of water dilutes themagnesium sulfate solution, leaving behind a salty residual solution onthe seawater side of the membrane 214. The residual solution may beremoved via line 219 and, optionally, returned to the sea.

The diluted magnesium sulfate solution is recovered from the module 210and transferred to the multi-stage flash distillation unit 212. In themulti-stage flash distillation unit 212, the second solution is heatedand introduced into an evaporation chamber, where it is subjected to apressure below its vapour pressure. The sudden reduction in pressurecauses boiling or flashing to occur. The flashed vapours may becondensed and separated from the remainder of the solution via line 220.The remaining solution is recycled to the module 210 via line 222. Aseries of evaporation chambers are employed so that the flashing orevaporation step occurs in multiple stages.

FIG. 5 depicts an apparatus for desalinating seawater using a processaccording to a fourth embodiment of the present invention. The apparatusof FIG. 5 is similar to the apparatus of FIG. 4. Thus, like numeralshave been used to designate like parts. Unlike the apparatus of FIG. 4,however, the apparatus of FIG. 5 comprises two modules 210 a and 210 bare used in series.

The first module 210 a comprises a semi-permeable membrane 214 a forseparating seawater 216 from a solution 218 a formed by dissolving aknown amount of magnesium sulfate in water. The second module 210 bcomprises a semi-permeable membrane 214 b for separating solution 218 afrom the first module 210 a from a solution 218 b formed by dissolving aknown amount of magnesium sulfate in water.

In use, seawater 216 is circulated through the module 210 a on one sideof the membrane 214 a, whilst magnesium sulfate solution 218 a iscirculated through the module 214 a on the opposite side of the membrane214 a. The magnesium sulfate solution in contact with the membrane 214 ahas a higher total dissolved salt (solute) concentration than theseawater 216. Thus, water flows from the seawater-side of the membrane214 a to the solution-side of the membrane 214 a by osmosis.

The flow of water across the membrane 214 a dilutes the magnesiumsulfate solution 218 a. The diluted solution 218 a is circulated throughthe module 210 b on one side of the membrane 214 b, whilst magnesiumsulfate solution 218 b is circulated through the module 210 b on theopposite side of the membrane 214 b. The solution 218 b in contact withthe membrane 214 b has a higher total dissolved salt (solute)concentration than the solution 218 a in contact with the membrane 214b. Thus, water flows across the membrane 214 b by osmosis to dilute themagnesium sulfate solution 218 b. The diluted solution 218 b isintroduced into multi-stage flash distillation unit 212 in the mannerdescribed with reference to FIG. 4.

As water flows across the membrane 214 b by osmosis, the magnesiumsulfate solution 218 a becomes increasingly concentrated and this isrecirculated to the first module 210 a.

FIG. 6 depicts an apparatus for desalinating seawater using a processaccording to a fifth embodiment of the present invention.

The apparatus 300 comprises two membrane module 310, 312, a thermalseparation unit 314 and a nanofiltration unit 316.

In use, seawater 318 is circulated through the first module 310 on oneside of the selectively permeable membrane 320, whilst magnesium sulfatesolution 322 is circulated through the module 310 on the opposite sideof the membrane 320. The magnesium sulfate solution in contact with themembrane 320 has a higher total dissolved salt (solute) concentrationthan the seawater 318. Thus, water flows from the seawater-side of themembrane to the solution-side of the membrane by osmosis.

The diluted magnesium sulfate solution is withdrawn from the module 310and introduced into the thermal separation unit 314. In the thermalseparation unit 314, the solution is cooled such that some of thedissolved solute precipitates out of solution at the base of the unit.The remainder of the solution has a reduced solute concentration and isintroduced into the second membrane module 314 via line 324. Thesolution 326 withdrawn from the base of the unit 314 has an increasedsolute concentration. This solution 326 is reused to extract water fromseawater in the membrane module 310.

In the second membrane module 312, the solution withdrawn via line 324is contacted with semi-permeable membrane 328. A magnesium sulfatesolution 330 having a higher total dissolved salts concentration thanthe solution withdrawn via line 324 is contacted with the opposite sideof the membrane. The difference in osmotic pressure on either side ofthe membrane 328 causes water to flow across the membrane 328 to dilutethe magnesium sulfate solution 330.

The diluted magnesium sulfate solution 330 is withdrawn and introducedinto the nanofiltration unit 316. The nanofiltration membrane in theunit 316 is used to separate solute components from the dilutedmagnesium sulfate solution. A portion 332 of the filtrate 332 isrecovered, whilst the remainder 334 is returned to the second module312.

The diluted solutions withdrawn from membrane modules 310, 312 may be atan elevated pressure, even when a pressure is not applied to induce theflow of water across the membrane 320, 328. This is because the flow ofwater occurs along a concentration gradient. This pressure may be usedto aid the subsequent extraction of water from the diluted solution. Forexample, the excess pressure may be used to drive the solution throughthe membrane in the nanofiltration unit 316.

The magnesium sulfate solution 322 circulated through the first membranemodule 310 may optionally be replaced with disodium hydrogen phosphate,(Na₂HPO₄.12H₂O). The solubility of sodium phosphate is more sensitive totemperature variation than magnesium sulfate. This may enhance theefficiency of the thermal separation unit 314.

The apparatus of FIG. 7 is similar to the apparatus of FIG. 6. Thus,like numerals have been used to designate like parts. Unlike theapparatus of FIG. 6, however, the apparatus of FIG. 7 further comprisesa multi-stage flash distillation unit 336.

In use, a portion 332 of the filtrate from the nanofiltration unit 316is recovered, whilst the remainder 334 of the filtrate is introducedinto the multi-flash distillation unit 336. In the multi-flashdistillation unit 336, water is separated from the filtrate 334 as avapour, which is condensed and recovered via line 338. Residual solution340 from the multi-stage flash distillation is returned to the secondmodule 312.

The apparatus of FIG. 8 is similar to the apparatus of FIG. 6. Thus,like numerals have been used to designate like parts. In use, however,the diluted magnesium sulfate solution 322 from the first membranemodule 310 is introduced directly into the second membrane module 312rather than the thermal separation unit 314.

In the second membrane module 312, the diluted magnesium sulfatesolution 322 is contacted with a semi-permeable membrane 328. A furthermagnesium sulfate solution 330 having a higher total dissolved saltsconcentration than the solution 322 is contacted with the opposite sideof the membrane 328. The difference in osmotic pressure causes water toflow across the membrane 328 to dilute the further magnesium sulfatesolution 330.

The diluted solution 330 from the second module 312 is introduced intothe thermal separation unit 314. In the thermal separation unit 314, thesolution is cooled such that dissolved solutes in the solutionprecipitate at the base of the unit 314. The remainder of the solutionhas a reduced solute concentration and is withdrawn from the top of theunit via line 324. This stream is introduced into the nanofiltrationunit 316, where it is filtered to produce a filtered water stream. Aportion 332 of the water stream is recovered, whilst the remainder isrecycled to the second module 312 via line 334.

The solution 326 from the base of the thermal separation unit 314 has anincreased solute concentration and is reused to extract water fromseawater in the second membrane module 312.

In the apparatus 400 of FIG. 9, there is provided a membrane module 410,a multi-stage flash distillation unit 412, a nanofiltration unit 416 anda thermal separation unit 414.

In use, seawater 418 is circulated through the first module 410 on oneside of a selectively permeable membrane, whilst magnesium sulfatesolution 420 is circulated through the module 410 on the opposite sideof the membrane. The magnesium sulfate solution 420 in contact with themembrane has a higher total dissolved salt (solute) concentration thanthe seawater 418. Thus, water flows from the seawater-side of themembrane to the solution-side of the membrane by osmosis.

A portion 422 of the diluted magnesium sulfate solution 420 isintroduced into the multi-stage flash distillation unit 412. In unit412, vapour extracted from the solution is condensed as a pure waterstream 424. The residual solution 426 emerging the unit 412 isintroduced into the thermal separation unit 414. In the thermalseparation unit 414, the solution 426 is cooled such that at least someof the dissolved solutes in the solution precipitate out of solution atthe base of the unit 414. The remainder 428 of the solution has areduced solute concentration and is filtered in the nanofiltration unit416 to provide a pure water stream 430. The solution from the base ofthe thermal separation unit 414 is withdrawn and returned to themembrane module 410.

The apparatus of FIG. 10 is similar to the apparatus of FIG. 9 and likenumerals have been used to designate like parts.

In use, the solution 420 is withdrawn from the membrane module 410 andintroduced into the thermal separation unit 414. In the thermalseparation unit 414, the solution 420 is cooled such that the dissolvedsolutes precipitate from the solution at the base of the unit 414. Theremainder of the solution has a reduced solute concentration. Thissolution 428 is withdrawn from the unit 414 and introduced into thenanofiltration unit 416. The solution at the base of the unit has ahigher solute concentration and is introduced into the multi-stage flashdistillation unit 412.

In the nanofiltration unit 416, the solution 428 is passed through ananofiltration membrane (not shown) that separates water from thesolution 428. A portion of the water is extracted via line 430, whilstthe remainder is removed via line 436 for further purification bymulti-stage flash distillation.

In the multi-stage flash distillation unit 412, water is extracted fromthe solution 434 as a vapour, which is condensed as a pure water stream424. The residual solution 426 from the multi-stage flash distillationunit is recycled to the membrane module 410.

1. A process for removing a solvent from a first solution, said processcomprising: a) positioning a selective membrane between the firstsolution and a second solution having a higher osmotic potential thanthe first solution, such that solvent from the first solution passesacross the membrane to dilute the second solution, and b) extractingsolvent from the second solution by passing the diluted second solutionthrough a nanofiltration membrane, wherein the nanofiltration membraneis cast as a skin layer on a support, and the separation properties ofthe nanofiltration membrane are controlled by the pore size andelectrostatic properties of the skin layer.
 2. A process as claimed inclaim 1, wherein the nanofiltration membrane is suitable for theseparation of components that are 0.001 to 0.01 microns in size.
 3. Aprocess as claimed in claim 1, wherein the second solution is preparedby introducing a known quantity of solute into a known quantity ofsolvent.
 4. A process as claimed in claim 1, which comprises dividingthe diluted second solution from step a) into a first portion and asecond portion, extracting solvent from the first portion by passing thefirst portion through the nanofiltration membrane of step b), andextracting solvent from the second portion by crystallization and/ordistillation.
 5. A process as claimed in claim 4, wherein the residuefrom the nanofiltration step b) is treated by a crystallization and/ordistillation technique.
 6. A process is claimed in claim 5, wherein thecrystallization and/or distillation technique is selected frommulti-flash distillation, multi-effect distillation, mechanical vapourcompression, MED-thermo compression and rapid spray distillation.
 7. Aprocess as claimed in claim 1, wherein the second solution is an aqueoussolution comprising at least one of magnesium sulfate (MgSO₄.6H₂O orMgSO₄.7H₂O), sodium sulfate (Na₂SO₄.10H₂O), calcium chloride (CaCl₂.2H₂Oor CaCl₂.6H₂O), potassium alum (24H₂O), disodium hydrogenphasphate(Na₂HPO₄.12H₂O), glucose, fructose and/or sucrose.
 8. A process asclaimed in claim 1, wherein the solvent of the second solution is thesame as the solvent of the first solution.
 9. A process as claimed inclaim 1, wherein the solvent of the second solution is water.
 10. Aprocess as claimed in claim 1, wherein the first solution is a wastestream from an industrial or agricultural process or a domestic waterstream.
 11. A process as claimed in claim 1, wherein the first solutionis a saline solution.
 12. A process as claimed in claim 11, wherein thesaline solution is seawater or brackish water.
 13. A process as claimedin claim 1, wherein the elevated pressure induced in the second solutionby the influx of solvent from the first solution is used to assist inthe extraction of solvent from the second solution.
 14. A process asclaimed in claim 1, wherein after solvent from the first solution passesacross the membrane to dilute the second solution, the diluted secondsolution is contacted with one side of a further selective membrane anda further solution having a higher osmotic potential than the dilutedsecond solution is contacted with the other side of the membrane, suchthat solvent from diluted second solution passes across the membrane todilute the further solution.
 15. A process as claimed in claim 1,wherein the second solution contains an additive selected fromanti-scaling agents, corrosion inhibitors, anti-fouling agents anddisinfectants.
 16. A process as claimed in claim 15, wherein said secondsolution is circulated in a closed loop, such that said additives arereused.
 17. A process as claimed in claim 1, wherein the selectivemembrane of step a) has an average pore size of 5 to 50 Angstroms.
 18. Aprocess as claimed in claim 1, wherein the membrane has an average poresize of at least 10 Angstroms and the second solution contains solutespecies that are too large to pass through the pores of the membrane.19. A process as claimed in claim 2, wherein the solute species in thesecond solution comprises at least one cationic species and/or at leastone anionic species that is larger than the average pore size of themembrane. 20-22. (canceled)